System and method for multi-spectral mr imaging near metal

ABSTRACT

A system and method for multi-spectral MR imaging near metal include a computer programmed to calculate an MR pulse sequence comprising a plurality of RF pulses configured to excite spins in an imaging object and comprising a plurality of volume selection gradients and determine a plurality of distinct offset frequency values. For each respective determined offset frequency value, the computer is programmed to execute the MR pulse sequence having a central transmit frequency and a central receive frequency of the MR pulse sequence set to the respective determined offset frequency value. The computer is also programmed to acquire a three-dimensional (3D) MR data set for each MR pulse sequence execution and generate a composite image based on data from each of the acquired 3D MR data sets.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation-in-part and claims priority toU.S. patent application Ser. No. 12/016,331 filed Jan. 18, 2008.

BACKGROUND OF THE INVENTION

Embodiments of the invention relates generally to magnetic resonance(MR) imaging and, more particularly, to MR imaging near metal.

When a substance such as human tissue is subjected to a uniform magneticfield (polarizing field B₀), the individual magnetic moments of thespins in the tissue attempt to align with this polarizing field, butprocess about it in random order at their characteristic Larmorfrequency. If the substance, or tissue, is subjected to a magnetic field(excitation field B₁) which is in the x-y plane and which is near theLarmor frequency, the net aligned moment, or “longitudinalmagnetization”, M_(Z), may be rotated, or “tipped”, into the x-y planeto produce a net transverse magnetic moment M_(t). A signal is emittedby the excited spins after the excitation signal B₁ is terminated andthis signal may be received and processed to form an image.

When utilizing these signals to produce images, magnetic field gradients(G_(x), G_(y), and G_(z)) are employed. Typically, the region to beimaged is scanned by a sequence of measurement cycles in which thesegradients vary according to the particular localization method beingused. The resulting set of received NMR signals are digitized andprocessed to reconstruct the image using one of many well knownreconstruction techniques.

The use of MR in musculoskeletal (MSK) diagnostics is a rapidly growingfield. Arthroplasty is the surgical placement of implants. Thepopulation of patients having some form of metal implant is quite largeand growing rapidly. MR has significant capabilities in assisting thediagnosis of implant revisions. Using magnetic resonance imaging toassist in clinical diagnostics of MR-compatible arthroplastic implants,however, has proven a fundamentally challenging problem. Most materialsthat are robust and durable enough to be utilized for bone replacementswill have magnetic properties that, when placed in a typical B₀ magneticfield, induce extraneous fields of amplitude and spatial variation thatare large compared to the field offsets utilized in conventional spatialencoding. Accordingly, these materials can introduce distortions in themain magnetic field resulting in an inhomogeneous magnetic field.

While the signal loss induced by these field gradients can largely beregained through the use of Hahn spin-echoes, the distortion theyproduce in both the readout and slice directions are drastic and aretypically unacceptable for clinical evaluation. Despite thesechallenges, MRI has been shown to be quite useful in the diagnosis ofdegenerative conditions in arthroscopic patients. In particular, MRI hasbeen used to screen perioprosthetic soft tissues, diagnose osteolysis,and visualize implant interfaces. These diagnostic mechanisms benefitsignificantly from visual information near implant interfaces.Unfortunately, artifacts induced by the implants in conventional MRIimages are most severe near the implant interfaces.

A proposed approach to reducing MRI artifacts induced by implants is 2DFSE imaging using View-Angle Tilting (VAT). Though this approach canimprove in-plane distortions at the cost of significant image blurring,it does not address distortions in the slice-selection direction. Nearthe most paramagnetic of utilized metallic implants, distortions in theslice-selection direction can almost completely disfigure 2D MR images.While a slice-distortion correction of VAT images in the slice directionhas been proposed, its use is limited because it does not correctsignal-pileup effects of image distortion.

It would therefore be desirable to have a system and method capable ofreducing image artifacts near or around implant interfaces. It wouldfurther be desirable to improve clinical diagnostic access to regions ofinterest near or around implant interfaces.

BRIEF DESCRIPTION OF THE INVENTION

According to one aspect of the invention, a magnetic resonance (MRI)apparatus comprises an MRI system having a plurality of gradient coilspositioned about a bore of a magnet, and an RF transceiver system and anRF switch controlled by a pulse module to transmit RF signals to an RFcoil assembly to acquire MR images. The MRI apparatus also includes acomputer programmed to calculate an MR pulse sequence comprising aplurality of RF pulses configured to excite spins in an imaging objectand comprising a plurality of volume selection gradients and determine aplurality of distinct offset frequency values. For each respectivedetermined offset frequency value, the computer is programmed to executethe MR pulse sequence having a central transmit frequency and a centralreceive frequency of the MR pulse sequence set to the respectivedetermined offset frequency value. The computer is also programmed toacquire a three-dimensional (3D) MR data set for each MR pulse sequenceexecution and generate a composite image based on data from each of theacquired 3D MR data sets.

According to another aspect of the invention, a method of magneticresonance (MR) imaging comprises calculating a plurality of 3D MR dataacquisitions, each 3D MR data acquisition comprises a plurality of RFpulses configured to excite spins in an imaging object; and a pluralityof volume selection gradients. The method also comprises determining adistinct central frequency for each of the plurality of 3D MR dataacquisitions and performing the plurality of 3D MR data acquisitions,each 3D MR data acquisition having a central transmit frequency and acentral receive frequency set to the distinct central frequencydetermined therefor. A composite image is generated from the pluralityof 3D MR data acquisitions.

According to another aspect of the invention, a non-transitory computerreadable storage medium having stored thereon a computer programcomprising instructions which when executed by a computer cause thecomputer to calculate a first 3D MR acquisition comprising a pluralityof RF pulses configured to excite spins in an imaging object andcomprising a plurality of volume selection gradients. The instructionsalso cause the computer to set a center transmission frequency and acenter reception frequency of the first 3D MR acquisition equal to afirst center frequency offset, to execute the first 3D MR acquisition toacquire a first set of 3D MR data, and to calculate a second 3D MRacquisition comprising a plurality of RF pulses configured to excitespins in an imaging object and comprising a plurality of volumeselection gradients. The instructions further cause the computer to seta center transmission frequency and a center reception frequency of thesecond 3D MR acquisition equal to a second center frequency offsetdifferent than the first center frequency offset, to execute the second3D MR acquisition to acquire a second set of 3D MR data; and toreconstruct a composite image based on the first and second sets of 3DMR data.

Various other features and advantages will be made apparent from thefollowing detailed description and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate embodiments presently contemplated for carryingout the invention.

In the drawings:

FIG. 1 is a schematic block diagram of an exemplary MR imaging systemincorporating embodiments of the present invention.

FIG. 2 is a flowchart of an MR imaging technique according to anembodiment of the invention.

FIG. 3 is a technique for constructing a magnetic field map according toan embodiment of the invention.

DETAILED DESCRIPTION

An apparatus and method is provided that acquires multiple 3D MR datasets where the center transmission frequency and the center receptionfrequency of each 3D MR data acquisition are set to an offset frequencythat is distinct for each 3D MR data set. A single image is constructedfrom the 3D MR data sets having reduced artifacts and reduced imagedistortion.

An apparatus and method is provided that acquires multiple 3D MR datasets where the center transmission frequency and the center receptionfrequency of each 3D MR data acquisition are set to an offset frequencythat is distinct for each 3D MR data set. A single image is constructedfrom the 3D MR data sets having reduced artifacts and reduced imagedistortion.

Referring to FIG. 1, the major components of an exemplary magneticresonance imaging (MRI) system 10 incorporating embodiments of thepresent invention are shown. The operation of the system is controlledfrom an operator console 12 which includes a keyboard or other inputdevice 13, a control panel 14, and a display screen 16. The console 12communicates through a link 18 with a separate computer system 20 thatenables an operator to control the production and display of images onthe display screen 16. The computer system 20 includes a number ofmodules which communicate with each other through a backplane 20 a.These include an image processor module 22, a CPU module 24 and a memorymodule 26 that may include a frame buffer for storing image data arrays.The computer system 20 is linked to archival media devices, permanent orback-up memory storage or a network for storage of image data andprograms, and communicates with a separate system control 32 through ahigh speed serial link 34. The input device 13 can include a mouse,joystick, keyboard, track ball, touch activated screen, light wand,voice control, or any similar or equivalent input device, and may beused for interactive geometry prescription.

The system control 32 includes a set of modules connected together by abackplane 32 a. These include a CPU module 36 and a pulse generatormodule 38 which connects to the operator console 12 through a seriallink 40. It is through link 40 that the system control 32 receivescommands from the operator to indicate the scan sequence that is to beperformed. The pulse generator module 38 operates the system componentsto carry out the desired scan sequence and produces data which indicatesthe timing, strength and shape of the RF pulses produced, and the timingand length of the data acquisition window. The pulse generator module 38connects to a set of gradient amplifiers 42, to indicate the timing andshape of the gradient pulses that are produced during the scan. Thepulse generator module 38 can also receive patient data from aphysiological acquisition controller 44 that receives signals from anumber of different sensors connected to the patient, such as ECGsignals from electrodes attached to the patient. And finally, the pulsegenerator module 38 connects to a scan room interface circuit 46 whichreceives signals from various sensors associated with the condition ofthe patient and the magnet system. It is also through the scan roominterface circuit 46 that a patient positioning system 48 receivescommands to move the patient to the desired position for the scan.

The gradient waveforms produced by the pulse generator module 38 areapplied to the gradient amplifier system 42 having Gx, Gy, and Gzamplifiers. Each gradient amplifier excites a corresponding physicalgradient coil in a gradient coil assembly generally designated 50 toproduce the magnetic field gradients used for spatially encodingacquired signals. The gradient coil assembly 50 forms part of a magnetassembly 52 which includes a polarizing magnet 54 and a whole-body RFcoil 56. In an embodiment of the invention, RF coil 56 is amulti-channel coil. A transceiver module 58 in the system control 32produces pulses which are amplified by an RF amplifier 60 and coupled tothe RF coil 56 by a transmit/receive switch 62. The resulting signalsemitted by the excited nuclei in the patient may be sensed by the sameRF coil 56 and coupled through the transmit/receive switch 62 to apreamplifier 64. The amplified MR signals are demodulated, filtered, anddigitized in the receiver section of the transceiver 58. Thetransmit/receive switch 62 is controlled by a signal from the pulsegenerator module 38 to electrically connect the RF amplifier 60 to thecoil 56 during the transmit mode and to connect the preamplifier 64 tothe coil 56 during the receive mode. The transmit/receive switch 62 canalso enable a separate RF coil (for example, a surface coil) to be usedin either the transmit or receive mode.

The MR signals picked up by the multi-channel RF coil 56 are digitizedby the transceiver module 58 and transferred to a memory module 66 inthe system control 32. A scan is complete when an array of raw k-spacedata has been acquired in the memory module 66. This raw k-space data isrearranged into separate k-space data arrays for each image to bereconstructed, and each of these is input to an array processor 68 whichoperates to Fourier transform the data into an array of image data. Thisimage data is conveyed through the serial link 34 to the computer system20 where it is stored in memory. In response to commands received fromthe operator console 12, this image data may be archived in long termstorage or it may be further processed by the image processor 22 andconveyed to the operator console 12 and presented on the display 16.

FIG. 2 shows a technique 70 for MR imaging near or around patientmetallic implants according to an embodiment of the invention. In anembodiment of the invention, technique 70 will be described with respectto 3D Fast Spin Echo (FSE) MR imaging; however, it is contemplated thattechnique 70 may also apply to 3D spin-echo sequences and other 3D MRacquisition techniques. In an embodiment of the invention, computersystem 20 may be programmed to perform technique 70. Technique 70 beginswith configuring a pulse train of an MR acquisition pulse sequence atblock 72 to be used during each MR acquisition of the MR imaging toacquire a 3D MR data set. The pulse sequence excites spins in an imagingobject such that MR data may be acquired therefrom. The shape of RFpulses of the pulse train within the MR acquisition pulse sequence isalso configured at block 72. In an embodiment of the invention, aGaussian pulse shape is used as the shape for the RF pulses of the pulsetrain. In another embodiment of the invention, the pulse shape used maybe based on a spatial-spectral pulse shape or on the shape of a hard orsquare pulse.

At block 74, imaging bandwidths for the MR acquisitions are determined.An excitation pulse bandwidth for the MR acquisition pulse sequence tobe used for acquiring MR data is determined as well as a bandwidth ofutilized refocusing pulses. The utilized refocusing pulse bandwidth isdetermined to be equal to or less than the bandwidth of the excitationpulse. A receiver bandwidth for the receive coil array used to acquireMR data during the MR acquisition pulse sequence is set to a bandwidthlarger than that typically used in 3D FSE imaging. In an example, thereceiver bandwidth is set to +/−125 kHz. It is contemplated that thereceiver bandwidth may also be set to a value greater than +/−125 kHz.In the described technique, off-resonance readout distortion is limitedto frequency offsets contained in the RF refocusing band. Setting thereceiver bandwidth accordingly helps to minimize this residual readoutdistortion in reconstructed images.

A resonance interval is determined at block 76 that represents an offsetfor both the center resonance frequency for transmission and the centerresonance frequency for reception between sets of acquired MR data.According to an embodiment of the invention, the resonance interval isless than the bandwidth of the utilized refocusing pulses. At block 78,a resonance interval sequence is determined for acquiring 3D MR datasets. The resonance interval sequence includes distinct offset frequencyvalues, or B₀ values, to which central transmission and centralreception resonance frequencies are set during MR acquisition. In anembodiment of the invention, the resonance interval sequence includes anoffset frequency value of zero. Additional values in the resonanceinterval sequence include multiples of the resonance interval. Forexample, the resonance interval sequence may include values for thecentral transmission and central reception resonance frequencies to beset to each 1 kHz offset step in the range −7 kHz to +7 kHz.

In an embodiment of the invention, the resonance interval sequence isset to interleave or interlace the offset frequency values such thatsequential MR acquisitions based on the offset frequency values do notacquire MR data with the central transmission and central receptionresonance frequencies set to sequential offset frequency values. Forexample, an interleaved resonance interval sequence with a 1 kHzresonance interval (or offset step) in the range −7 kHz to +7 kHz mayhave the following order: [−7, 1, −5, 3, −3, 5, −1, 7, −6, 0, −4, 6, −2,4, 2 kHz]. Accordingly, neighboring values in the resonance intervalsequence are separated by more than the offset step of 1 kHz.Interleaving the resonance interval sequence in this manner reducesinteraction between 3D MR data acquisitions in an imaging scan. Asdescribed further below, each offset frequency value in the resonanceinterval sequence is used as the central transmission and receptionfrequency for a different 3D MR data acquisition. In one embodiment, anMR imaging scan (or protocol) may be configured such that a first set ofacquisitions uses a resonance interval sequence with the offsetfrequency values [−7, 1, −5, 3, −3, 5, −1, 7 kHz] during a single scanand such that a second set of acquisitions uses a resonance intervalsequence with the offset frequency values [−6, 0, −4, 6, −2, 4, 2 kHz]during another single scan. The resonance interval sequence valueslisted above are illustrative only and do not limit the invention. Otherand different orders and values for the resonance interval sequencevalues are considered and are within the scope of the invention.

At block 80, the amplitude of a plurality of volume selection gradientsis determined or configured. In one embodiment, the volume selectiongradients are applied in the slice selection direction. While the sliceselection direction is referred to herein as being in the z-direction,it is illustrative only and does not limit the invention. Other anddifferent slice selection directions are considered and are within thescope of the invention.

Determination of the amplitude is based on a desired field-of-view (FOV)and spectral coverage. For example, consider an acquisition with N_(b)spectral bins and a bin separation of Ω_(b), kHz. The excited z FOV,according to an embodiment, is desired to be restricted to ΔZ cm. Thiscan be accomplished by applying the pulse sequence excitations under a zor slice select gradient of G_(z) amplitude according to the followingexpression:

$\begin{matrix}{{G_{z} = \frac{2\pi \; N_{b}\Omega_{b}}{{\gamma\Delta}\; Z}},} & \left( {{Eqn}.\mspace{14mu} 1} \right)\end{matrix}$

where N_(b) is the number of spectral bins, Ω_(b) is the gap betweencenter frequencies of spectral bins, γ is the gyromagnetic ratio ofprotons, and Gz corresponds to a gradient strength or amplitude ofvolume selection gradients in a slice selection direction.

At block 82, the central transmission and central reception resonancefrequencies for a 3D MR data acquisition are both set to one of thevalues in the resonance interval sequence, in particular, the centraltransmission frequency and the central reception frequency for theacquisition are set to the same offset frequency value. 3D MR data isacquired at block 84 using the scan parameters and sequences configuredand determined in the previous steps of technique 70. In an embodimentof the invention, the 3D MR data is acquired using non-parallel imagingtechniques. The 3D MR data may be acquired via multi-channel RF coil 56of FIG. 1 or via another multi-channel receive coil. However, it iscontemplated that parallel imaging techniques such as ARC, and the likemay also be used and that multiple multi-channel receive coils may beused to acquire the 3D MR data. At block 86, it is determined if another3D MR data acquisition should be performed. If all the offset frequencyvalues in the resonance interval sequence have not been used 88, thenprocess control returns to block 82 for setting the central transmissionand central reception resonance frequencies for the next 3D MR dataacquisition to another of the offset frequency values in the resonanceinterval sequence and at block 84 3D MR data for another 3D MR data setis acquired as described above. If all the offset frequency values inthe resonance interval sequence have been used 90, an image isreconstructed at block 92 for each MR data set acquired, resulting in acollection of images. Each image is reconstructed using knownreconstruction techniques.

A final, single composite image is constructed at block 94. In anembodiment of the invention, the composite image is constructed usingthe maximum intensity projection (MIP) of each pixel from the collectionof images. A pre-determined pixel location in each image of thecollection of images is used to determine which image contains thegreatest intensity projection for the pre-determined pixel location. Thegreatest intensity projection value is then used for the same locationin the final composite image.

In another embodiment, the final, single composite image may beconstructed at block 94 using iterative reconstruction based on amagnetic field map. FIG. 3 shows a technique 96 for constructing amagnetic field map according to an embodiment of the invention. At block98, a plurality of reconstructed MR images are selected. In oneembodiment, the plurality of reconstructed MR images are retrieved froman image storage location such as memory 26 shown in FIG. 1 or anothercomputer readable storage medium. The plurality of reconstructed imagesmay be generated using the technique described above with respect toFIG. 2. In another embodiment, the plurality of reconstructed MR imagesmay be generated on-the-fly. For example, the plurality of MR images maybe generated using the technique described above with respect to FIG. 2.At block 100, the pixels for each of the plurality of MR images areexamined to determine, for each pixel location, which image of theplurality of MR images has the maximum intensity. At block 102, eachpixel location in the magnetic field map is assigned the offsetfrequency value, or B₀ value, to which the central transmission andcentral reception resonance frequencies are set to in the imagedetermined to have the maximum intensity for the corresponding pixellocation. For example, for a given magnetic field map pixel location,the image acquired with the central transmission and central receptionresonance frequencies set to 3 kHz may have the maximum intensity forthe corresponding pixel location. Accordingly, the value of 3 kHz isused for the given magnetic field map pixel location. For an adjacentmagnetic field map pixel location, it may be determined that the imageacquired with the central transmission and central reception resonancefrequencies set to −4 kHz may have the maximum intensity for thecorresponding pixel location. Accordingly, the value of −4 kHz is usedfor the adjacent magnetic field map pixel location.

Returning to FIG. 2, in another embodiment, the composite image may beconstructed at block 94 using a sum-of-squares method where the MR datasets are acquired using Gaussian-shaped pulses. One skilled in the artwill recognize that the steps of technique 70 may be performed inanother order than that described and that such is within the scope ofembodiments of the invention.

Embodiments of the invention allow for improved MR imaging near oraround metallic implants such that artifacts and image distortion arereduced. Embodiments of the invention are applicable in MR imaging wheresignificant heterogeneity of the B₀ magnetic field exists.

A technical contribution for the disclosed method and apparatus is thatis provides for a computer implemented method for MR imaging ininhomogeneous magnetic fields.

One skilled in the art will appreciate that embodiments of the inventionmay be interfaced to and controlled by a computer readable storagemedium having stored thereon a computer program. The computer readablestorage medium includes a plurality of components such as one or more ofelectronic components, hardware components, and/or computer softwarecomponents. These components may include one or more computer readablestorage media that generally stores instructions such as software,firmware and/or assembly language for performing one or more portions ofone or more implementations or embodiments of a sequence. These computerreadable storage media are generally non-transitory and/or tangible.Examples of such a computer readable storage medium include a recordabledata storage medium of a computer and/or storage device. The computerreadable storage media may employ, for example, one or more of amagnetic, electrical, optical, biological, and/or atomic data storagemedium. Further, such media may take the form of, for example, floppydisks, magnetic tapes, CD-ROMs, DVD-ROMs, hard disk drives, and/orelectronic memory. Other forms of non-transitory and/or tangiblecomputer readable storage media not list may be employed withembodiments of the invention.

A number of such components can be combined or divided in animplementation of a system. Further, such components may include a setand/or series of computer instructions written in or implemented withany of a number of programming languages, as will be appreciated bythose skilled in the art. In addition, other forms of computer readablemedia such as a carrier wave may be employed to embody a computer datasignal representing a sequence of instructions that when executed by oneor more computers causes the one or more computers to perform one ormore portions of one or more implementations or embodiments of asequence.

Therefore, according to one embodiment of the invention, a magneticresonance (MRI) apparatus comprises an MRI system having a plurality ofgradient coils positioned about a bore of a magnet, and an RFtransceiver system and an RF switch controlled by a pulse module totransmit RF signals to an RF coil assembly to acquire MR images. The MRIapparatus also includes a computer programmed to calculate an MR pulsesequence comprising a plurality of RF pulses configured to excite spinsin an imaging object and comprising a plurality of volume selectiongradients and determine a plurality of distinct offset frequency values.For each respective determined offset frequency value, the computer isprogrammed to execute the MR pulse sequence having a central transmitfrequency and a central receive frequency of the MR pulse sequence setto the respective determined offset frequency value. The computer isalso programmed to acquire a three-dimensional (3D) MR data set for eachMR pulse sequence execution and generate a composite image based on datafrom each of the acquired 3D MR data sets.

According to another embodiment of the invention, a method of magneticresonance (MR) imaging comprises calculating a plurality of 3D MR dataacquisitions, each 3D MR data acquisition comprises a plurality of RFpulses configured to excite spins in an imaging object; and a pluralityof volume selection gradients. The method also comprises determining adistinct central frequency for each of the plurality of 3D MR dataacquisitions and performing the plurality of 3D MR data acquisitions,each 3D MR data acquisition having a central transmit frequency and acentral receive frequency set to the distinct central frequencydetermined therefor. A composite image is generated from the pluralityof 3D MR data acquisitions.

According to yet another embodiment of the invention, a non-transitorycomputer readable storage medium having stored thereon a computerprogram comprising instructions which when executed by a computer causethe computer to calculate a first 3D MR acquisition comprising aplurality of RF pulses configured to excite spins in an imaging objectand comprising a plurality of volume selection gradients. Theinstructions also cause the computer to set a center transmissionfrequency and a center reception frequency of the first 3D MRacquisition equal to a first center frequency offset, to execute thefirst 3D MR acquisition to acquire a first set of 3D MR data, and tocalculate a second 3D MR acquisition comprising a plurality of RF pulsesconfigured to excite spins in an imaging object and comprising aplurality of volume selection gradients. The instructions further causethe computer to set a center transmission frequency and a centerreception frequency of the second 3D MR acquisition equal to a secondcenter frequency offset different than the first center frequencyoffset, to execute the second 3D MR acquisition to acquire a second setof 3D MR data; and to reconstruct a composite image based on the firstand second sets of 3D MR data.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they have structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal languages of the claims.

1. A magnetic resonance (MRI) apparatus comprising: an MRI system havinga plurality of gradient coils positioned about a bore of a magnet, andan RF transceiver system and an RF switch controlled by a pulse moduleto transmit RF signals to an RF coil assembly to acquire MR images; anda computer programmed to: calculate an MR pulse sequence comprising aplurality of RF pulses configured to excite spins in an imaging objectand comprising a plurality of volume selection gradients; determine aplurality of distinct offset frequency values; for each respectivedetermined offset frequency value, execute the MR pulse sequence havinga central transmit frequency and a central receive frequency of the MRpulse sequence set to the respective determined offset frequency value;acquire a three-dimensional (3D) MR data set for each MR pulse sequenceexecution; and generate a composite image based on data from each of theacquired 3D MR data sets.
 2. The MRI apparatus of claim 1 wherein thecomputer is further programmed to determine an amplitude of theplurality of volume selection gradients based on a desired field-of-view(FOV) and a desired spectral coverage.
 3. The MRI apparatus of claim 2wherein the computer is programmed to determine the amplitude of theplurality of volume selection gradients based on the equation:${G_{z} = \frac{2\pi \; N_{b}\Omega_{b}}{{\gamma\Delta}\; Z}},$where N_(b) corresponds to a number of spectral bins, Ω_(b), iscorresponds to gap between center frequencies of spectral bins, γcorresponds to a gyromagnetic ratio of protons, and G_(z), correspondsto a gradient amplitude of the plurality of volume selection gradientsin a given direction, z.—phase-selection should be changed to volumeselection in the rest of the claims
 4. The MRI apparatus of claim 1wherein the plurality of volume selection gradients comprises: a firstgradient configured to be applied in a slice direction during anexcitation RF pulse of the plurality of RF pulses; a second gradientconfigured to be applied in the slice direction during a refocusing RFpulse of the plurality of RF pulses; and a third gradient configured tobe applied in the slice direction during a readout gradient pulse of theMR pulse sequence.
 5. The MRI apparatus of claim 1 wherein the computeris further programmed to reconstruct an image for each of the 3D MR datasets.
 6. The MRI apparatus of claim 5 wherein the computer is furtherprogrammed to perform a maximum intensity projection technique on thereconstructed images to generate the composite image.
 7. The MRIapparatus of claim 5, wherein the computer is further programmed toperform a sum-of-squares technique on the reconstructed images togenerate the composite image.
 8. The MRI apparatus of claim 5 whereinthe computer is further programmed to generate a magnetic field map fromthe reconstructed images.
 9. The MRI apparatus of claim 8 wherein thecomputer is further programmed to perform iterative reconstruction usingthe magnetic field map to generate the composite image.
 10. The MRIapparatus of claim 1 wherein the computer is further programmed tointerleave offset frequency values used to acquire the plurality of 3DMR data sets such that sequential 3D MR data sets are acquired usingnon-sequential offset frequency values.
 11. A method of magneticresonance (MR) imaging comprising: calculating a plurality of 3D MR dataacquisitions, each 3D MR data acquisition comprising: a plurality of RFpulses configured to excite spins in an imaging object; and a pluralityof volume selection gradients; determining a distinct central frequencyfor each of the plurality of 3D MR data acquisitions; performing theplurality of 3D MR data acquisitions, each 3D MR data acquisition havinga central transmit frequency and a central receive frequency set to thedistinct central frequency determined therefor; and generating acomposite image from the plurality of 3D MR data acquisitions.
 12. Themethod of claim 11 further comprising determining an amplitude of theplurality of volume selection gradients based on a desired field-of-view(FOV) and a desired spectral coverage.
 13. The method of claim 12wherein determining the amplitude of the plurality of volume selectiongradients comprises determining the amplitude of the plurality of volumeselection gradients based on the equation:${G_{z} = \frac{2\pi \; N_{b}\Omega_{b}}{{\gamma\Delta}\; Z}},$where N_(b) corresponds to a number of spectral bins, Ω_(b), iscorresponds to gap between center frequencies of spectral bins, γcorresponds to a gyromagnetic ratio of protons, and G_(z), correspondsto a gradient amplitude of the plurality of volume selection gradientsin a slice selection direction.
 14. The method of claim 11 whereinperforming each of the plurality of 3D MR data acquisitions comprises:applying a first volume selection gradient of the plurality of volumeselection gradients in a slice direction during an excitation RF pulseof the plurality of RF pulses; applying a second volume selectiongradient of the plurality of volume selection gradients in the slicedirection during a refocusing RF pulse of the plurality of RF pulses;and applying a third volume selection gradient of the plurality ofvolume selection gradients in the slice direction during a readoutgradient.
 15. The method of claim 11 further comprising reconstructingan image for each of the plurality of 3D MR data acquisitions; andwherein generating the composite image comprises applying a maximumintensity projection technique on the reconstructed images.
 16. Anon-transitory computer readable storage medium having stored thereon acomputer program comprising instructions which when executed by acomputer cause the computer to: calculate a first 3D MR acquisitioncomprising a plurality of RF pulses configured to excite spins in animaging object and comprising a plurality of volume selection gradients;set a center transmission frequency and a center reception frequency ofthe first 3D MR acquisition equal to a first center frequency offset;execute the first 3D MR acquisition to acquire a first set of 3D MRdata; calculate a second 3D MR acquisition comprising a plurality of RFpulses configured to excite spins in an imaging object and comprising aplurality of volume selection gradients; set a center transmissionfrequency and a center reception frequency of the second 3D MRacquisition equal to a second center frequency offset different than thefirst center frequency offset; execute the second 3D MR acquisition toacquire a second set of 3D MR data; and reconstruct a composite imagebased on the first and second sets of 3D MR data.
 17. The computerreadable storage medium of claim 16 wherein the instructions that causethe computer to calculate the first and second 3D MR acquisitions causethe computer to: determine an amplitude of the plurality of volumeselection gradients based on the equation:${G_{z} = \frac{2\pi \; N_{b}\Omega_{b}}{{\gamma\Delta}\; Z}},$where N_(b) corresponds to a number of spectral bins, Ω_(b), iscorresponds to gap between center frequencies of spectral bins, γcorresponds to a gyromagnetic ratio of protons, and G_(z), correspondsto a gradient amplitude of the plurality of volume selection gradientsin a slice selection direction.
 18. The computer readable storage mediumof claim 16 wherein the instructions that cause the computer tocalculate the first and second 3D MR acquisitions cause the computer to:calculate a shape of the plurality of RF pulses based on a Gaussianpulse shape.
 19. The computer readable storage medium of claim 16wherein the instructions that cause the computer to reconstruct thecomposite image cause the computer to: construct a magnetic field mapbased on at least the first and second sets of 3D MR data; anditeratively reconstruct the composite image based on at least themagnetic field map and the first and second sets of 3D MR data.
 20. Thecomputer readable storage medium of claim 16 wherein the instructionsfurther cause the computer to: calculate a third 3D MR acquisitioncomprising a plurality of RF pulses configured to excite spins in animaging object and comprising a plurality of volume selection gradients;set a center transmission frequency and a center reception frequency ofthe third 3D MR acquisition equal to a third center frequency offset;execute the third 3D MR acquisition to acquire a third set of 3D MRdata; and interleave the center transmission and reception frequenciesof the first, second, and third 3D MR acquisitions such that the centertransmission and reception frequencies of one of the first and third 3DMR acquisitions is between the center transmission and receptionfrequencies of the other 3D MR acquisitions.